Veins, in the animal body, are membranous canals, destined for the purpose of re-conveying the blood from the arteries to the heart. They run chiefly by the sides of arteries, but more towards the surface ; and are, like these, composed of three membranes, namely, the interior coat, resembling the arterial ; the second, or cellular ; and the third, consisting of longitudinal fibres ; the whole being, however, of a more delicate texture, so that they are apt to rupture, in consequence of too great expansion. Farther, the veins are provided with numerous thin, semi-lunar valves, which prevent the return of the blood ; a circumstance that would otherwise frequently occur, from a want of muscular power in the venous system : hence, no pulsation can take place in these vessels. The blood which they receive from the arteries, flows but slowly to the heart; and is conveyed thither by the contractility of their membranes; the propulsion of the succeeding columns of that fluid from the arteries; the contraction of the muscles ; and by the act of respiration.
In cases of venesection by the lancet, the blood being generally taken from the veins, we deem it a duty, to caution our readers against resorting to unskilful hands, where such an operation becomes necessary ; as, from the contiguity of the arteries, as well as the nerves and tendons, serious injuries may be sustained, and which, in many cases, have proved fatal. - If, during blood-letting, the patient become faint, it will be advisable to admit fresh air into the room ; to give mild cordials ; and to by the person in a horizontal posture : by these means, the circulation will again be restored, and all farther inconvenience be obviated.
For an account of the varicose aneurism, an affection of the veins, we refer the reader to the article Aneurism.